This invention relates to a method of producing an iron-base dispersion-strengthened alloy tube, with the use of a rolling machine having grooved rolls and a mandrel, by cold rolling or warm rolling.
An iron-base dispersion-strengthened alloy is known to have a structure in which inert particles such as oxides, nitrides, carbonides, intermetallic compounds, etc. are dispersed uniformly in an iron matrix. The materials are alloys which maintain a high strength to a temperature range near a melting point of iron, and have been extensively and favorably used for piping materials employed in high temperature and high pressure conditions such as boiler tubes, piping of internal combustion and fuel cladding tubes for fast breeder reactors in nuclear power generation.
The iron-base dispersion-strengthened alloy have been produced by a so-called powder metallurgy method in which inert particles as described above and powder of iron-base alloy are mixed by, for example, a ball mill, formed and sintered. There are cases that additional hot working is provided after sintering to form products having predetermined dimensions. However, since the alloy of this type has less deformability, there is a serious problem due to the difficulty of achieving a suitable warm or cold working as well as hot working.
A tube product such as a boiler tube and a fuel cladding tube needs cold working or warm working for at least a final working step, from a viewpoint of dimensional accuracy. However, the iron-base dispersion-strengthened alloy is of difficult workability and, therefore, cold working and warm working of tubes will cause cracking on the surfaces, resulting in difficulty in realization of production.
Japanese Patent 2,564,826 discloses a method for producing a tube from a dispersion-strengthened alloy. In the method, at least three double-enveloping or hourglass shaped rolls which are supported to a cam groove inclined to an axis of a rolling tube are simultaneously pressed against the same circumferential surface of a raw tube for a tube and the rolls are reciprocated in an axial direction of the raw tube for the tube, so that the raw tube is subjected to oscillation-rolling in a radial direction of the raw tube for reducing a diameter of the raw tube. According to the Japanese Patent described, the method disclosed therein allows favorable production having pipes of a small or reduced size in thickness and diameter. The rolling machine used therein is reported to be a HPTR-type rolling machine.
Here, FIGS. 1(a) and 1(b) show a principle of a rolling method conducted by the HPTR rolling machine of a three-roll type, wherein FIG. 1(a) is a partly section side view seen from a side of a rolling line and FIG. 1(b) is an enlarged view in transversal section as seen from a front side of the rolling line.
With reference to FIGS. 1(a) and 1(b), in the rolling method by the three-roll type HPTR rolling machine, a rolling tube (raw tube) 2 with a mandrel 1 inserted therethrough is treated with diameter-reduction working and thickness-reduction working by a reciprocal movement of rolls 3 in an axial direction of the rolling tube to obtain rolled tube material (finished tube) 2-1 having a small size in both diameter and thickness. The roll 3 is a rotary body having a double enveloping or hourglass shaped body in a transversal sectional view parallel to the axis of the roll as illustrated in FIG. 1(b) and, therefore, the shape of a rolling surface 3-1 is substantially the same as the shape of a caliber formed by the rolls which is used in steel bar rolling, etc. and thus, the entire circumference of the rolls is of the same curvature and same depth, and the curvature is equivalent to a curvature of outer diameter do of the rolled tube material (finished tube) after the rolling procedure. When the rolls 3 are advanced along with an inclined cam groove 4 from a starting position R1of rolling to a finishing position R2(shown by dotted lines) as indicated by an arrow "f" in FIG. 1(a), the rolls 3 are pressed downward by the cam groove 4 in a radial direction of the rolling tube 2 to proceed with the workings of diameter reduction and thickness reduction.
In case that an iron-base dispersion-strengthened alloy is subjected to rolling by HPTR rolling method, a rolling reduction achieved by a single working is 20 percent, at most, as disclosed in the Japanese Patent described above. Accordingly, it is almost impossible or at least difficult to increase a ratio of an outer diameter of the raw tube at the time of starting the rolling relative to an outer diameter of the finished tube after the rolling procedure and, therefore, in order to prepare a small diameter tube product from a raw tube having a large outer diameter, repetition of a number of working steps is required, which results in an extraordinary reduction in production efficiency.
Here, the term "rolling reduction" Rd recited above intends to mean a value which is obtained by the following equation (1). ##EQU1##
In the equation (1), Do and Di are outer diameter and inner diameter, respectively, of the raw tube at the time of starting the working procedure, and do and di are outer diameter and inner diameter, respectively, of the finished tube immediately after completing the working process.
In the conventional HPTR rolling method, reasons for incapability of increasing the in rolling reduction by a single working procedure will be explained as set forth below.
FIGS. 2(a) and 2(b) show a contact condition or state between rolling tube and each of rolls at the time of tube production by the conventional three-roll type HPTR rolling machine, wherein FIG. 2(a) shows a state of contact at a start of the rolling procedure and FIG. 2(b) shows a state of contact at a finishing position of the rolling procedure.
At the start position of rolling (R.sub.1 in FIG. 1) as shown in FIG. 2(a), only edge portions 3-2 of the roll 3 are contact with the rolling tube and other portions are not contact with the same. As draft is initiated by an advancing movement of the rolls 3, the edge portions 3-2 cut into a surface of the rolling tube 2 and this will cause cracks to be formed on the surface of the rolling tube.
As the working proceeds, the contact between the roll surface and the tube is increased. However, for a certain instance, there is a phenomenon that the rolling is proceeded with the contact being limited or small-scaled between the roll surface and the rolling tube until the contact region is extended so that a bottom of the rolling surface 3-1 is contact with the rolling tube. Particularly in one case of the iron-base dispersion-strengthened alloy tube, there is less elongation in a circumferential direction and difficulty in deforming the surface in line with the shape of the caliber of the rolls during the working procedure and, therefore, it is likely that a phenomenon as described above occurs. If it is tried to use a raw tube having an increased diameter to thereby proceed working with a large rolling reduction, a width of the roll which contacts the rolling tube is relatively reduced, so that a space between the rolls (that is, a non-arresting portion of the rolling tube) 3-3 is increased. On the non-arresting portion, a tensile stress is generated in a circumferential direction on the outer surface of the rolling tube and this results in the occurrence of cracks on the surface. These are the reasons why the rolling reduction cannot be increased.